The installation of replacement joint prostheses involves surgically exposing and dislocating the joint, and cutting away or resecting one or both of the portions of the bones making up the joint. The most common prosthetic joint surgical procedures are total hip, knee and finger joint procedures. Knee and finger joint arthroplasty procedures are undertaken to correct for degeneration in the mating joint bones and cartilage caused by injury or disease processes. Healthy knee and finger joints enjoy a range of motion between a fully extended and a fully flexed position which is desirable to reproduce with the implanted replacement prosthesis. Moreover, it is desired to reattach and reposition tendons and the patella (or artificial patellar component) in the proper anatomic positions to provide strength and support of the replacement joint and to inhibit dislocations and prevent implant failures.
With respect to knee arthroplasty, typically both components of the knee joint need to be replaced. Knee arthroplasty requires that the ends of the fibial bone and femoral bone (condyles) of the patient be resected, that is, cut and shaped in order to receive the respective tibial and femoral components of the prosthesis in proper alignment. In the placement of the tibial and femoral components, it is also often necessary to bore the respective bones to permit them to receive implant stems extending from each component.
The resection of the femoral and tibial bones typically results in the removal of cartilage and bone. The subsequent implantation of a standard size prosthesis could result in an overall shortening or lengthening of the joint.
One way surgeons avoid this problem is by implanting thicker knee components to compensate for the lost bone. For example, some femoral component designs are available in a variety of thicknesses, in order to obtain for various final sizes. For example, Howmedica, Inc. sells a "P.C.A." revision total knee system which has small, medium and large femoral components. The P.C.A. revision total knee system includes a series of tibial base inserts that fit upon the plate of the tibial component that are provided in various thicknesses and sizes. The surgeon has the choice of selecting from these available femoral and tibial components in order to approximate the over all dimensions of the patient's original knee joint. Frequently, surgeons choose the thickest tibial insert that will fit between the tibial plate and femoral component after the latter components are surgically attached so that the tendons extending therebetween are tightened up and little play is found in the joint as the knee is flexed.
The use of such a thick femoral component or tibial insert or an over-sized component to compensate for bone loss can, however, result in poor ligament balance, improper positioning of the joint line, and shifting of the patella position with a change of arc of motion (leading to dislocation). This, in turn, can cause poor tracking of the patella on the femoral component in the range of motion between full extension and the extent of flexure permitted by the design of the prosthesis. If the patella is improperly positioned, it may shift laterally with respect to the femoral component and dislocate off the front of the knee to a lateral position, causing the patient considerable distress due to pain and instability. This patellar instability will frequently occur if the joint is lengthened, whether or not the patellar surface is replaced. The patellar misalignment occurs primarily due to the relative lengthening of the joint, which increases the are of motion over the knee joint.
In addition, using the thicker or oversized components may also lengthen the patient's leg, causing an imbalance with the other leg that is obviously undesirable. Relative lengthening of the joint also results in loss of full extension and/or flexion.
In the presence of severe deterioration of the tibia, it is also known to fill in gaps in the resected tibia inferior to the plate of the tibial component so that the plate may be raised and a thinner tibial insert employed. See, for example, the article by Peter J. Brooks, M.D., et al, entitled "Tibial Component Fixation in Deficient Tibial Bone Stock," Clinical Orthopedics and Related Research, pp. 302-308, Vol. 184, April 1984. Similarly, femoral shims adapted to be positioned in the superior recess of the femoral component are disclosed in U.S. Pat. No. 4,731,086. Thus, the surgeon has available a number of techniques and components for approximating the original dimensions of the knee joint or for dimensionally correcting imbalanced dimensions in the arthroplasty procedure.
At present, however, surgeons typically rely on gross artatomic dimensional measurements of leg length and visual approximations of the knee joint. They further rely on trial and error insertions of tibial inserts of differing thicknesses in making the final determination of the components to be permanently implanted. See, for instance, "Asymmetrical Buildup and Prosthesis Sizing", pages 67-73, in The Technique of Total Knee Arthroplasty, K. A. Krackow, 1990, C. V. Mosby Co., which describes the pitfalls associated with the use of x-ray films as a preoperative means to determine approximate sizing requirements.
Thus, an object of this invention is to provide a simple, reproducible technique and apparatus for use in such arthroplasty procedures of a bone joint movable between an extended and a flexed position for simplifying the selection of these components, to regain accurate joint height and pateliar excursion length, and achieving a higher degree of consistency and predictability in the post-surgical operation of the joint prosthesis.